When children engage in social aggression, they hurt peers by damaging their friendships and social status. Social aggression includes behaviors such as social exclusion (verbal and non-verbal), friendship manipulation, and malicious gossip. This Independent Scientist Award (K02) application seeks funding to support Marion K. Underwood, Ph.D., to investigate more fully the early developmental origins of social aggression and the psychosocial outcomes associated with engaging and being victimized by social aggression. Dr. Underwood is a child clinical psychologist with expertise in social aggression in the middle childhood and adolescent age ranges and experience in measuring social aggression using a variety of methods (creative observational techniques, sociometric nominations, and parent-, teacher-, and self-report questionnaires). She is the Principal Investigator of a longitudinal project currently in its third year, following a sample of 281 children from ages 9 -14 to investigate developmental origins and outcomes of social aggression. Short-term career development goals include augmenting quantitative skills to analyze these longitudinal data to maximum advantage, and increasing knowledge of preschool development and building a collaborative team to conduct a longitudinal study of social aggression beginning with 2-3-year-old children. The long-term goal of this research program is to understand better the role of social aggression in the development of psychopathology in girls and boys, to determine whether reducing social aggression might be helpful in preventing the development of externalizing disorders, internalizing problems, personality disorders, and eating disorders, and in promoting positive adjustment and achievement. Careful analyses of how social aggression unfolds, both developmentally and in real time, will guide the future development of prevention and intervention programs to reduce social aggression among girls and boys.